In a typical printing press, such as the digital printing press described, for example, in U.S. Pat. Nos. 5,357,617 and 5,385,092 assigned to Presstek Inc. of New Hampshire, U.S.A, the drum forms part of the press. In another application, the drum may be a drum of a computer to plate image setter.
In the above mentioned patents assigned to Presstok Inc., the focusing apparatus comprises a plurality of IR (infrared) lasers diode, known in the art. The light emitted by each laser diode is focused by a corresponding focusing lens. Thus, a large number of lenses are required, whereby the complexity and the cost of the focusing apparatus increase.
In a second type of focusing apparatus, which is described in U.S. Pat. Nos. 5,109,460 and 5,168,288 assigned to Eastman Kodak Company (Kodak) of Rochester, N.Y., USA and schematically illustrated in FIG. 1 to which reference is now made; the thermal printer 10 includes a movable focusing apparatus 12 moving in the direction indicated by arrows 2 to affect line by line scanning on a drum 14 rotating about a longitudinal axis as indicated by arrow 16.
Optical focusing apparatus used in printing systems such as lithographic offset printing generally comprises optical fibers connected to a lens system. The optical focusing apparatus is fixed at a pre-determined distance from the printing drum. To adjust the focus, the whole lens system or one of the lenses within the system are moved to the required focusing position by moving the lenses relative to the light source.
The focusing apparatus, of U.S. Pat. Nos. 5,109,460 and 5,168,288 use an autofocus mechanism. The focusing apparatus 12 comprises an array of IR laser diodes 22A-22E. Each laser diode 22A-22E is attached to a corresponding optical fiber 24A-24E in a pigtail type attachment, the light emitting ends of the plurality of fiber optics are aligned at 26.
In this arrangement, the light from all IR laser diodes 22 is focused onto the drum 14 by a single optical assembly 28. The optical assembly 28 comprises a stationary lens assembly 30 and a movable focusing lens or lens assembly 32. In FIG. 1, an exemplary light path 34C is shown for the light emitted by laser diode 22C to affect exposure of the medium mounted on drum 14 at exposure spot 36C.
One drawback of IR laser diodes is that in order to obtain the output power required to expose the IR sensitive medium, fiber optics with a large diameter, typically 100 microns, and a large numerical aperture, typically larger than 0.2, are required. Moreover, in order to meet quality requirements of the exposed image, the focusing lens images the output of the fiber optics with a demagnification ratio of 3, thus leading to a numerical aperture of 0.6 towards the image plane.
Since the numerical aperture of the focusing lens is high, an autofocusing mechanism is designed to compensate for changes in the distance between the surface of the-printing member and the aligned light emitting end 26 of the fiber optics 24A-24E. This autofocusing compensation mechanism includes the movable lens or lens assembly 32 which is movable between stationary lens assembly 30 and the drum 14 as indicated by an arrow 4.
In the illustrated example, lens 32 moves to a second position 32' (dashed lines) as indicated by arrow 4 so as to change the optical path from 34 to 34' in order to expose the light sensitive medium in exposure spot 36C' thus compensating for the movement of the medium on the drum 14 as indicated by location 14' of the drum.
A drawback of autofocusing optical assemblies, in particular ones which provide an accuracy of the exposed spot in terns of location and spot size on the order of microns is their cost and complexity and the fact that they are prone to mechanical failures.
In U.S. Pat. No. 5,221,997 assigned to the present applicants, an alternative system is described for a lens assembly having a plurality of linearly arranged elements which permits each element to be individually adjusted from one access location of the assembly by means of an adjusting element
One of the main disadvantages of existing focusing systems is that the more accurate the focusing required the more expensive the system needs to be. Furthermore, the adjustment of one of the lenses in a system may mis-align the overall lens arrangement.